1. Field of the Invention
The present invention relates to a method and an apparatus for grinding ball grooves which are formed in each of two members for sliding one of them relative to the other via balls.
2. Description of Related Art
Conventionally, it is normal practice to grind ball grooves by feeding a grind stone along a longitudinal axis of the ball groove where an axis of rotation of the grind stone is positioned to perpendicular across the longitudinal axis of the ball groove (see, for example, Japanese Published Unexamined Patent Application No. 26816/1985).
Referring to FIG. 7, an angle .alpha. of contact is taken as an angle to be formed by an axial line of rotation of a ball B and that line of diameter of the ball B which passes through a point of contact of the ball B with a ball groove Wa. Then, the ball groove Wa formed in a workpiece W must be ground to a shape in which the angle .alpha. of contact falls within a predetermined range.
In the above-described conventional method of grinding, it is so arranged that the grinding of the ball groove is performed by using a grind stone which has been formed into a cross-sectional shape which coincides with the shape of a normal ball groove in which the angle of contact becomes a predetermined value. In this case, in order to confirm the accuracy of the shape of the grind stone, the following arrangement is adopted. Namely, a test machining or grinding is performed on a test piece made of a relatively soft material such as carbon or the like. When the shape of the groove obtained by this test machining is outside a predetermined allowable range, the grind stone is reformed to a predetermined shape.
Furthermore, a clearance groove is formed at the bottom of the ball groove. Therefore, when the ball groove is ground, out of the external peripheral surface of the grind stone, only those portions of the grind stone which serve to grind the side surfaces of the ball groove get worn. As a result, the shape of the groove becomes gradually narrower, with a consequent smaller angle of contact. In order to cope with this situation, in the conventional method, the grind stone is changed after every predetermined number of times of grinding the ball groove.
In the above-described conventional method, when the shape of the groove to be obtained by the test machining becomes narrower with a consequent smaller angle of contact, a large degree of correction is required in reforming the grind stone. At the same time, the previously performed reforming work will become utterly useless, resulting in a higher grinding cost. Further, since the grind stone must be changed frequently, the cost of grinding also becomes expensive.
In view of the above-described disadvantages, the present invention has an object of providing a method and an apparatus for grinding ball grooves in which, even if the accuracy in shape of the grind stone may be somewhat poorer, the grind stone can be used without reforming and in which the frequency of change in the grind stone can be decreased, with the result that the cost of grinding can be largely reduced.
According to one aspect of the present invention, the foregoing and other objects are attained by a method of grinding ball grooves formed in a workpiece with a grind stone, the method comprising the step of feeding the grind stone along a longitudinal axis of the ball groove wherein an axis of rotation of the grind stone is inclined at an angle relative to a plane which is perpendicular to the longitudinal axis.
According to another aspect of the present invention the foregoing and other objects are attained by an apparatus for grinding ball grooves formed in a workpiece comprising: an apparatus bed which is movable along a longitudinal axis of the ball groove relative to a work holder for holding a workpiece; a supporting bed which is provided on the apparatus bed; a grind stone holder which rotatably mounts on a front end thereof a grind stone for grinding the ball groove; driving means for rotating the grind stone, the grind stone holder and the driving means being mounted on the supporting bed; and means for adjustably supporting the supporting bed on the apparatus bed such that the supporting bed is adjustable in inclination about an axis line of inclination which is perpendicular to an axis line of rotation of the grind stone and which passes through the center of the grind stone.
If the supporting bed of the present invention apparatus is inclined on the apparatus bed, the direction of the axis line of rotation of the grind stone which is attached to the supporting bed via the grind stone holder varies. As a result, the axis line of rotation of the grind stone is inclined relative to the plane which perpendicularly crosses the axis line of the ball groove of the workpiece which is held on the work holder. If the apparatus bed is moved relative to the work holder, the grind stone will be fed as it is in the direction of the axial line of the ball groove to thereby grind the ball groove. Here, the shape of the ball groove will be broadened depending on the angle of inclination of the axis line of rotation, with the consequent increase in the angle of contact. Therefore, even if the shape of the groove at the time of test machining by feeding the grind stone in a direction perpendicular to the axis of rotation of the grind stone (the shape should then correspond to the cross section of the grind stone) has become narrower than the normal shape of the ball groove, a predetermined angle of contact can be obtained if the angle of inclination of the axis of rotation of the grind stone is set, by the adjustment of inclination of the supporting bed, to a value depending on the deviation of the shape of the groove from the normal shape. It follows that the grind stone can be used as it is without reforming, resulting in a reduction in cost. Further, if the inclination angle of the axis of the grind stone is increased by a predetermined angle after every predetermined number of times of grinding the ball groove, the decrease in the angle of contact due to wear of the grind stone can be compensated. Therefore, the angle of contact can be made to fall within a predetermined allowable range without changing the grind stone, with the result that the frequency of change in the grind stone can be decreased.
When the grind stone has worn, the surface of the grind stone is likely to become rougher due to dropping or disappearing of grinder particles, with the result that grinding lines or streaks are likely to be left on the ball groove. However, since the grind stone comes to rotate along or in parallel to the plane which crosses askew or at an angle to the axis line of the ball groove due to the inclination of the axis of rotation of the grind stone, the streaks will be smoothed by the feeding of the grind stone in the direction of the axis of the ball groove. The surface roughness of the ball groove will therefore not be impaired.
If the position of the grind stone is moved or shifted by the inclination of the supporting bed, the relative positional relationship of the work holder and the apparatus bed must be corrected. According to the present invention apparatus, however, the axis of inclination of the supporting bed is arranged to pass through the center of the grind stone and, therefore, the position of the grind stone will not be moved even if the supporting bed is inclined. The above-described correction is thus not required and the workability is improved.
In order to grind the ball grooves which are formed on an internal circumference of the workpiece, the grind stone holder must be formed in an elongated shape which can be inserted into the inside of the workpiece.
In this case, if there are provided a pair of arms which extend in the same direction as the grind stone holder at a distance therebetween in the direction of the axis of inclination such that the workpiece and the work holder can be disposed between the arms, and if the supporting bed is pivotally supported on the apparatus bed via the pair of arms, the following advantages can be obtained. Namely, even if the supporting bed is pivotally supported on the axis of inclination which passes through the center of the grind stone which is rotatably mounted on the front end of the grind stone holder, there will occur no interference of the supporting bed with the workpiece and the work holder. Further, the supporting rigidity of the supporting bed relative to the apparatus bed can also be secured.